# The Application of Saline–Alkali-Tolerant Growth-Promoting Endophytic Bacteria for Enhancing the Saline–Alkali Tolerance of Alfalfa

## TL;DR

Researchers found that certain bacteria can help alfalfa grow better in salty and alkaline soils, boosting yield and quality for sustainable agriculture.

## Contribution

Identification of SYM-15, a highly effective saline–alkali-tolerant endophytic bacterial strain, for improving alfalfa growth and nutritional quality.

## Key findings

- SYM-15 increased alfalfa yield by 34.5% and protein content by 13.3% while reducing fiber content.
- Inoculation with SYM-15 significantly enhanced plant height, root length, and antioxidant enzyme activities in saline–alkali soils.
- SYM-15 reduced harmful compounds like hydrogen peroxide and malondialdehyde in alfalfa plants.

## Abstract

Soil salinity and alkalinity limit crop growth worldwide. Alfalfa, an important livestock feed, grows poorly in such soils. We isolated beneficial bacteria from alfalfa roots growing in saline–alkali soil in northeast China. Four bacterial strains were tested for their ability to help alfalfa grow under stress. These bacteria produced growth-promoting substances that stimulated root development and helped plants absorb nutrients. In pot and field experiments, the bacteria significantly improved plant height, root biomass, and overall yield. The best strain was SYM-15, which increased alfalfa yield by 34.5% and protein content by 13.3% while reducing fiber content. This research study shows that naturally occurring bacteria can be developed into environmentally friendly bio-fertilizers to improve crop production on marginal lands, contributing to sustainable agriculture.

Saline–alkali stress is a severe abiotic factor that limits plant growth and development. Endophytic bacteria can improve plant tolerance to such stress through various mechanism, including osmoregulatory substance accumulation and antioxidant enzyme activity. In this study, four saline–alkali-tolerant endophytic strains, designated SYM-2, SYM-4, SYM-9, and SYM-15, were isolated from the roots of alfalfa grown in saline–alkali soil. Though 16S rDNA sequencing, morphological observations, and physiological–biochemical characterization, the strains were identified as closely related to Bacillus cereus, B. thuringiensis, B. halotolerans, and Pantoea agglomerans, respectively. These strains demonstrated the ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, siderophores, and indole-3-acetic acid (IAA), as well as solubilizing phosphorus. Under saline–alkali conditions, inoculation with these strains significantly increased alfalfa growth parameters. Plant height increased by 4.07–33.90% and root length by 7.49–27.94%, and fresh and dry weight (both above and below ground) increased compared with the control. Strain SYM-15 showed the highest promoting effects, increasing plant height by 33.90%, root length by 27.94% and shoot dry weight by 59.26%. Additionally, root activity increased by 11.23–40%, proline content by 19.09–129–87%, and soluble protein by 7.71–42.49%, and the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were significantly elevated across treatments. At the same time, inoculation reduced the levels of hydrogen peroxide (H2O2), superoxide anion (O2−), and malondialdehyde (MDA). Compared with the control and other treatments, including SYM-9, the peroxidase activity and superoxide dismutase activity of alfalfa significantly increased after the SYM-15 treatment, while hydrogen peroxide content, phosphorus content, and neutral detergent fiber and acid detergent fiber contents decreased (p < 0.05). Therefore, SYM-15 plays an important role in promoting growth and represents a promising, high-quality strain resource for the large-scale development of microbials aimed at improving alfalfa tolerance under saline–alkali conditions.